Since it is extremely difficult to determine the final grade, elevation or pitch of the blacktop when sewers are installed, construction workers come as close as possible to the target elevation and then establish the final elevation of the manhole cover housing by inserting a spacer ring composed of concrete. Concrete spacers are, however, subject to breakage in shipment. U.S. Pat. No. 4,759,656 proposes that an elastomer be used for this purpose. An elastomer such as rubber, although capable of forming a tight seal against concrete or iron, is expensive. Rubber can also creep over time. By contrast, it is an object of the present invention to provide a spacer ring composed of scrap plastic, such as scrap beverage containers, and thereby provide a very inexpensive product but one with a provision for achieving a tight seal even though the plastic body of the spacer itself is rigid.
It has been proposed, for example in U.S. Pat No. 4,763,449, to utilize plastic resin, e.g., polyethylene, as a gasket for the manhole cover itself but the gasket merely encloses the edge of a flange and is not suited to support a manhole cover housing. In addition, it is not self-aligning nor stackable (capable of being mated together in a stack one on top of another).
In view of these shortcomings, it is an object of the present invention to provide a stackable manhole housing height adjustment ring which (a) requires substantially less resin than previously used, (b) is self-aligning with respect to adjacent rings or other objects, (c) will hold the manhole cover housing in a fixed position, (d) although itself rigid, has a provision for establishing a reliable seal between adjacent rings and between itself and adjacent concrete or iron surfaces, and (e) has an optional feature for changing the pitch of the top ring.
These and other more detailed and specific objects of the present invention will be better understood by reference to the following figures and detailed description which illustrate by way of example but a few of the various forms of the invention within the scope of the appended claims.